Description: The proposed studies focus on the contribution of the cochlear-efferent system in the susceptibility to noise and the phenomenon of "sound conditioning." Several hypotheses are proposed to determine whether the susceptibility of the ear can be experimentally manipulated through sound conditioning, if the cochlear-efferent system is involved in sound conditioning and whether auditory or systemic factors are involved in the process. In addition, the molecular bases of protection from sound-overexposure will be explored. The functional measure employed in the experiment is the 2f1-f2 DPOAE and its alteration by presentation of a pure tone presented to the opposite ear. The change in DPOAE with binaural stimulation is proposed as a measure of "efferent strength," which appears to vary among individual animals and humans. Three hypotheses are proposed. In the first, awake rabbits will be used as subjects to determine if the efferent system plays a role in 'sound conditioning', whether the effect is ear-specific and whether other systemic factors are involved. The second hypothesis tests the extent to which "sound conditioning" is apparent in mice that have varying susceptibility to noise-induced hearing loss. The third hypothesis tests whether susceptibility to hearing loss or the prevention from it is associated with specific molecules in the cochlea.